Typically when you first start lifting your gains are very impressive but growth doesn't continue at that fast pace. Now you need to continuous shock your muscles into new growth, do this by constantly changing your routine. Do new exercises, experiment with different reps, occasionally do forced reps or drop sets. You could also prioritize a particular muscle group in you workout and blast it. Never let your muscles get used to the same routine. Train with high intensity and don't do too many sets per body part.
You should also experiment with different split routines, training 5 days a week doesn't allow for much recovery time, even though you are young.
As for nutrition it would help if you posted a sample day of you diet. Don't rely too heavily on shakes for protein - make sure you eat low fat dairy protein and quality white meat, lots of complex carbs and good fats.
Supplements like whey protein and creatine are the fundamentals, although I cycle my creatine intake so I don't over tax my kidneys. Others on the board will have great advice on which supplements work effectively for them.